Cassini/Huygens at Saturn and Titan III Posters
Presiding: A Kliore, Jet Propulsion Laboratory, California Institute of Technology; R M Nelson, Jet Propulsion Laboratory, California Institute of Technology
P21C-01 0830h
The Surface Properties of Titan as Revealed by Cassini VIMS
The Cassini Visual Infrared Mapping Spectrometer is an instrument that returns data between 0.34 and 5.1 microns. Spatial resolution can be as high as 0.25 mradians in the 1-5 micron range and 0.17 mradians in the visible. By the end of May 2005 the Cassini spacecraft will have accomplished 5 close, targeted flybys of Titan, ranging in distances (at closest approach) from 1025 to 4009 km. The VIMS instrument is uniquely suited to studying the surface of Titan because its thick, opaque nitrogen-methane atmosphere exhibits clarity at selected "windows" that are well-distributed throughout the near infrared. The spectral reflectance at these windows has been studied to derive the surface properties of the satellite. A simple empirical model has employed to remove the haze, which adds substantial opacity even in the windows. The surface scattering properties have been fit to a macroscopic roughness model, and to a Henyey-Greenstein equation to derive the single particle phase function. Many areas of Titan have scattering properties - and presumably surface properties - similar to the other icy satellites of Saturn. The over-all shape of Titan's spectrum through the atmospheric windows is consistent with a mixture of water-ice and organics (tholins), although this simple picture cannpt explain some details. Work funded by the National Aeronautics and Space Administration.
P21C-02 0830h
Radiative Transfer in Primordial Atmosphere of Titan
In light of Huygens measurements, we present our improved model of thermal and photochemical evolution of Titan's atmosphere. Atreya et. al (1978) demonstrated that photolysis of ammonia on primordial Titan is capable of producing a nitrogen atmosphere substantially thicker than that measured by Voyager. E. Wilson (2001) carried this calculation one step further by including methane and water vapor explicitly in the ammonia photochemistry model, and arrived at a preliminary estimate of time required to accumulate different amounts of nitrogen. However, both models assumed an isothermal atmosphere. Since chemistry leading up to nitrogen occurs in the stratosphere, both the thermal structure and saturation effects are important for determining the time constants and amounts of nitrogen production. In this presentation, we discuss preliminary results of a radiative equilibrium model for the primordial middle and lower atmosphere of Titan. It includes CH4, NH3 and H2O in solar proportions for its initial composition, and CH4-CH4 pressure induced absorption, which presently controls the thermal structure in the troposphere. The temperature in the stratosphere is controlled by the haze, and we explore the effects of a haze layer at various altitudes for accelerating conversion of ammonia to nitrogen. Furthermore, we include the effects of enhanced solar flux during the T-Tauri phase, which could speed up both the loss of nitrogen and conversion of ammonia to nitrogen. We are in the process of coupling the radiative transfer model to a comprehensive photochemical model (Wilson and Atreya, 2004) to access the roles of trace species other than those included in this calculation.
P21C-03 0830h
Whistlers Observed in the Magnetosphere of Saturn
Whistlers generated by lightning have been observed in the magnetosphere of Saturn by the Cassini Radio and Plasma Wave Science Investigation (RPWS). Two whistlers were observed as the spacecraft flew over the rings on July 1, 2004, and the third was observed on October 28, 2004, during the inbound pass of orbit A at a radial distance of 6.19 RS (Saturn radii). Of the three, the third has the best signal-to-noise ratio and is the main subject of this presentation. The whistler has a good fit to the well-known Eckersley law for the dispersion of whistlers, with a dispersion constant of 81.3 Hz1/2 sec. Since to a first approximation the whistler energy follows the planetary magnetic field line, the lightning that caused the whistler must be located at a relatively high latitude, roughly 66 degrees in this case. It is not known whether the causative lightning was located in the northern or southern hemisphere. However, the location of the spacecraft at 12.4 degrees north latitude and the relatively large dispersion suggest that the whistler passed through the dense equatorial plasma torus. If so, the lightning would be located in the southern hemisphere. Based on the measured dispersion and local electron density at the spacecraft, which was about 6.5 cm-3, the effective path length through the torus can be determined and is about 2.17 RS. Although not completed at the present time, we plan to use a Gaussian scale height model of the plasma torus to determine the north-south thickness of the torus from the measured dispersion. This model can then be compared with other estimates of the torus thickness, such as can be obtained from the plasma temperature using a centrifugal potential model. As more whistlers are detected during the Cassini mission, we should eventually be able to determine the north-south thickness of the torus as a function of radial distance.
P21C-04 0830h
Electron Density Measurements in Saturn's Inner Magnetosphere
During Saturn orbit insertion and during the periapsis portion of the three subsequent Cassini orbits, the Radio and Plasma Wave Science (RPWS) instrument obtained near-continuous measurements of the upper hybrid resonance emissions on both the inbound and outbound portions of the orbits. Independent verification of the upper hybrid frequency has also been obtained from measurements of the RPWS Sounder instrument on Cassini. Electron densities have been derived from these frequency measurements with a high degree of accuracy for Saturn's inner magnetosphere outside the rings (2.2 < RS < 10). Preliminary electron density profiles for these early orbits show a predictably strong radial dependence, varying as (1/R)α where α ranges from about 3 to 4. In addition to the radial dependence, latitudinal variations are also observed. We will develop a simple centrifugal potential model of the plasma density for Saturn's inner magnetosphere as a function of radial distance with a latitudinal factor.
P21C-05 0830h
Cassini Observations of Auroral Hiss-Like Emissions Near Saturn's Rings
A well-defined funnel-shaped electric field emission was detected by the plasma wave instrument onboard the Cassini spacecraft during its pass over the rings of Saturn on July 1, 2004. The funnel-shaped frequency-time characteristic is very similar to a type of whistler-mode emission called 'auroral hiss' that is commonly observed in Earth's auroral region. This is the first detection of such emissions at Saturn. Using a dipole magnetic field and a simple electron density model based on the electron density profile measured by Cassini, ray tracing computations have been performed to explain the funnel-shaped frequency-time spectrum. It is found that the source is located very close to the B ring at a distance of about 1.80 RS (Saturn radii), near the synchronous rotation point in the rings. Terrestrial auroral hiss emissions are known to be caused by field-aligned low-energy (100 eV to 1 keV) electron beams associated with the auroral current system. The existence of similar emissions originating from near Saturn's rings suggest that an electrodynamic interaction between the rings and Saturn's co-rotating magnetospheric plasma may be driving a field-aligned system of beams and currents similar to those occurring in Earth's auroral regions.
P21C-06 0830h
Loss from Saturn's E Ring through ion pick-up
When the neutrals that compose Saturn's E ring are exposed to solar radiation or impacting energetic particles, they become ionized at a rate of the order of 10-9/s. Within Saturn's magnetosphere, any newly created ions will be accelerated by the electric field associated with the corotating magnetized plasma. If the energy the ions gain in this pick-up process is sufficiently great, they will then begin to produce magnetic oscillations at a characteristic frequency. When Pioneer 11 and Voyager 1 passed through this region of the saturnian system, their magnetometers both observed intervals of such waves whose frequencies point to water-group ion sources. The magnetometer onboard the Cassini spacecraft has also observed these water-group ion cyclotron waves. The range of coverage from the multiple passes allow us to analyze their variation in space and time. The water-group ions that create these waves represent a loss of material from the neutral E ring so, by studying the ion cyclotron waves as a proxy for mass loading, we can quantify the rates of erosion of the E ring through ionization and magnetospheric transport.
P21C-07 0830h
Where did the Spokes go?
The intermittently appearing bright (in forward scattered light) or dark (in backscattered light) radial features above Saturn's B ring ("spokes") were discovered by the Voyager imaging experiment. These features represented a major challenge to our understanding of celestial mechanics in planetary rings. It was clear early on that, in addition to gravity, electromagnetic forces must play a role in the formation and evolution of spokes. Because of the competing theories about spoke formation, Cassini was scheduled to make a large number of dedicated observations. However, contrary to our high expectations no spokes to date have been observed by Cassini. Initially this was thought to be due to the geometry of the observations, however by now it is clear that the spokes are indeed absent in the current epoch. In this talk we argue that the absence of spokes is due to a seasonal modulation of the plasma environment in the rings. The photoelectron density above the rings is determined by solar irradiance, hence the elevation angle of the Sun. Following the argument of Nitter et al. (1998), we use simple physical model to follow the trajectory and the charge of small grains lofted from the rings into the plasma sheath. These calculations verify that the probability of a small grain rising above the photoelectron sheath is a function of the plasma density and, hence the solar elevation angle. Based on this seasonal variation of the spoke activity we expect that spokes will reappear towards the end of Cassini's main mission in 2008.
P21C-08 0830h
Dust Impacts Detected by the Cassini Radio and Plasma Wave Instrument Near Saturn's Ring Plane on July 1, 2004
During the inbound and outbound passes of the Cassini spacecraft through Saturn's ring plane on July 1, 2004, the Radio and Plasma Wave Science (RPWS) instrument detected many small particles striking the spacecraft. When a small particle strikes the spacecraft at a high velocity it is instantly vaporized, producing a small cloud of plasma that expands outward from the impact site. As the plasma cloud sweeps over the RPWS electric field antennas it produces a voltage pulse, the amplitude of which is believed to be proportional to the mass of the impacting particle. Two types of measurements are made: waveform measurements from the x-axis dipole antenna, and spectrum measurements from the w-axis monopole antenna. The waveform measurements provide a determination of the impact rate and the relative mass distribution, and the spectrum measurements provide a determination of the rms particle mass. The impact rate at both ring plane crossings provides a good fit to the sum of two Gaussians, with a peak impact rate of about 1000 per second (the exact value depends on the voltage threshold used), and a north-south thickness (at half the peak rate) of about 300 km. The mass distribution depends on the distance from the ring plane, varying from about m-2 near the ring plane, at z=0 ±100 km, to m-4 well away from the ring plane, at z=500 ±100 km, where z is the north-south distance from the ring plane. We are still working on estimates of the rms mass and the typical size of the dust grains, and will present preliminary results on these estimates.
P21C-09 0830h
Molecular Oxygen Ions Within Saturn's Inner Magnetosphere as Observed by Cassini: Initial Results
We will present initial results of our analysis of molecular oxygen ions within Saturn's inner magnetosphere as observed by the Cassini Plasma Spectrometer (CAPS) experiment. As reported in Young et al. (2005) the O2+ is a minor species in the magnetosphere, but is dominant over the main rings (Tokar et al., 2005). This analysis, confined outside Mimas' L shell, will be built around the work of Sittler et al. (2005) who computed fluid parameters of protons and water group ions within Saturn's inner plasmasphere. A non-linear analysis of the composition data will be used. We will constrain the analysis by requiring the O2+ to be comoving with the water group ions. This will allow us to make estimates of ion density and temperature for the O2+ along the various Cassini trajectories through Saturn's inner magnetosphere. Molecular oxygen is important because it can be produced by gas phase processes, but can also be a signature of the decomposition of icy surfaces by radiolysis and photolysis (Johnson et al 2003; 2004). Therefore, similar to the observation of an ozone-like feature on Dione and Rhea (Noll et al. 1997), the observation of molecular oxygen ions can be a marker for the radiation-induced erosion of ice grains and icy bodies within Saturn's magnetosphere. Our results will examine the formation and redistribution of molecular oxygen within the inner magnetosphere.
P21C-10 0830h
An Assessment of Electric Currents Flowing in Saturn's Magnetosphere and Their Modeling
As the magnetic dipole and rotation axes of Saturn are aligned, spacecraft traveling through the Kronian magnetosphere do not experience diurnal changes caused by the motions of the current systems. Therefore, the strengths of current systems can only be assessed by careful modeling. We combine observations from the Pioneer, Voyager and Cassini spacecraft and model the strengths of the currents flowing in various regions of the Kronian magnetosphere. We use techniques and methods used by modelers for the earth's magnetosphere to develop new models of Saturn's magnetospheric field. We use Tsyganenko and Peredo [1994] models of disk-shaped current sheets to model the magnetic field of Saturn's current sheet. The tilt, and hinging of the current sheet is introduced by using the general deformation technique [Tsyganenko, 1998]. The shielding field from the magnetopause for the equatorial current sheet and the internal field is specified by Cartesian and cylindrical harmonics, respectively. Additional modules specify the field of the radial current system which reinforces corotation on the outflowing plasma. and the interconnection field between solar wind IMF and the magnetosphere. A comparison of model field with the observation is provided. Finally, we extract the profiles of the currents from the modeled parameters.